Cleaning solution and method for cleaning semiconductor substrates after polishing of copper film

ABSTRACT

A cleaning solution for cleaning a semiconductor substrate is formed by mixing an amount of citric acid, an amount of ammonium fluoride, and an amount of hydrogen fluoride in deionized water. In one embodiment, the amount of citric acid is in a range from about 0.09% by weight to about 0.11% by weight, the amount of ammonium fluoride is in a range from about 0.4% by weight to about 0.6% by weight, the amount of hydrogen fluoride is in a range from about 0.09% by weight to about 0.11% by weight, and the cleaning solution has a pH of about 4. A method for cleaning a semiconductor substrate having a polished copper layer in which a concentrated cleaning solution is mixed with deionized water proximate to a scrubbing apparatus also is described.

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/955,393, now U.S. Pat. No. 6,165,956, entitled “Methods andApparatus for Cleaning Semiconductor Substrates After Polishing ofCopper Film,” filed Oct. 21, 1997. The disclosure of this priorityapplication is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and apparatus for processing andcleaning a substrate, and more specifically to methods and apparatus forcleaning semiconductor substrates after polishing of copper films.

2. Background Information

In the manufacture of advanced semiconductor devices, copper (Cu) isbeginning to replace aluminum (Al) as the material for metallization. Cuhas become desirable due to its lower resistivity and significantlyimproved electromigration lifetime, when compared to Al.

One process for Cu metallization uses a dual damascene approach. Asillustrated in FIG. 1a, a dielectric layer 110 is deposited above asubstrate 100. Dielectric layer 110 may be made up of materials such assilicon dioxide. Vias and/or trenches 120 are then formed in thedielectric layer 110, as illustrated in FIG. 1b. Vias/trenches 120 maybe formed, for example, using dry etching techniques. Next, a thin layerof barrier material (barrier layer) 130, for example, tantalum (Ta),titanium (Ti), or titanium nitride (TiN) is deposited as illustrated inFIG. 1c. After barrier layer 130 is deposited the vias/trenches 120 arefilled with copper (Cu) layer 140, as illustrated in FIG. 1d. Cu layer140 may be deposited using well known deposition techniques, forexample, chemical vapor deposition (CVD), physical vapor deposition(PVD), or electroplating. In order to isolate the copper interconnects,as illustrated in FIG. 1e, the excess copper layer 140 and barrier layer130 must be removed.

One method for removing the excess copper layer 140 and barrier layer130 is polishing the surface of the substrate, for example, polishingusing chemical mechanical polishing (CMP). In a CMP process, thesemiconductor substrate is polished with a slurry containing abrasiveparticles, such as alumina particles, and an oxidant, such as hydrogenperoxide. In the CMP process, contaminants are introduced which includeparticles and/or metal contamination on the copper layer 150, dielectricsurface 160, and in the dielectric subsurface 165.

Regardless of how the CMP process is performed, the surface ofsemiconductor substrate must be cleaned of contaminants. If not removed,these contaminants may affect device performance characteristics and maycause device failure to occur at faster rates than usual. Cleaning thesemiconductor substrate after chemical mechanical polishing of coppermay be necessary to remove such contaminants from the copper layer anddielectric layers.

One method for cleaning the semiconductor substrate after polishing ofthe copper layer is brush scrubbing. Brush scrubbing, whethersingle-sided or double-sided brush scrubbing, is the industry standardfor cleaning oxide and tungsten CMP applications. However, there areseveral problems associated with applying brush scrubbing to post copperCMP cleaning.

One such problem is brush loading. During the CMP process, the topsurface of the copper layer may be oxidized and forms copper oxide, forexample copper oxide (Cu₂O or CuO) or copper hydroxide (Cu(OH)₂). Inbasic or neutral pH cleaning environments, the copper oxide or copperhydroxide does not dissolve and may be transferred to the brushes, thusloading the brushes. The contaminated (or loaded) brushes may thentransfer the copper oxide or copper hydroxide contaminants tosubsequently processed substrates during cleaning.

For tungsten and other oxide applications, brush loading could becurtailed by adding a dilute ammonium hydroxide (NH₄OH). In the presenceof NH₄OH, part of the copper oxide may form Cu(NH₃)²⁺ complex and may bedissolved; however, due to the high pH environment, the dilute ammoniumhydroxide has been found to be insufficient to prevent brush loading ofcopper oxide. Additionally, it has been found that scrubbing with diluteammonium hydroxide also causes etching of the copper layer and may causeserious surface roughening.

Brush loading may also occur when alumina particles are used in thecopper CMP process. In neutral or inorganic acid (e.g. HCl) cleaningenvironments, there is an electrostatic attraction between aluminaparticles and the silicon dioxide surface which makes it difficult toremove the alumina particles from the surface of the dielectricmaterial. Because of the electrostatic attractive force, the aluminaparticles may also adhere to the brush and cause another brush loadingproblem with similar effects to those discussed above.

Yet another problem caused by the CMP process is that the surface andsubsurface of the dielectric layer may become contaminated duringpolishing with metal from the copper layer and barrier layer as well asother contaminants from the slurry. During the CMP process,contaminants, especially metal contaminants, may penetrate into thedielectric layer up to approximately 100 angstroms (Å) from the surface.Again, these contaminants may affect device performance characteristicsand may cause device failure.

Thus, what is needed is a cleaning environment and methods for cleaningpost copper CMP substrates that alleviate the problems of brush loadingwithout affecting the quality of the copper and dielectric layers.Furthermore, what is needed is a cleaning environment and methods forcleaning post copper CMP substrates that have the capability of removingsurface and subsurface contaminants from the copper and dielectriclayers.

SUMMARY OF THE INVENTION

In one aspect of the invention, a cleaning solution for cleaning asemiconductor substrate is provided. The cleaning solution may be formedby mixing an amount of citric acid, an amount of ammonium fluoride, andan amount of hydrogen fluoride in deionized water. In one embodiment,the amount of citric acid is in a range from about 0.09% by weight toabout 0.11% by weight, the amount of ammonium fluoride is in a rangefrom about 0.4% by weight to about 0.6% by weight, and the amount ofhydrogen fluoride is in a range from about 0.09% by weight to about0.11% by weight, and the cleaning solution has a pH of about 4.

In another aspect of the invention, a method for cleaning asemiconductor substrate having a polished copper layer is provided. Inthis method, a concentrated cleaning solution is provided. Thesemiconductor substrate having the polished copper layer is placed in ascrubbing apparatus. In one embodiment, one unit volume of theconcentrated cleaning solution is mixed with 20 unit volumes ofdeionized water proximate to the scrubbing apparatus to obtain acleaning solution. The semiconductor substrate is scrubbed in thescrubbing apparatus in the presence of the cleaning solution.

Additional features and benefits of the present invention will becomeapparent from the detailed description, figures, and claims set forthbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the accompanying figures in which:

FIG. 1a illustrates a semiconductor substrate having a dielectric layerdeposited thereon.

FIG. 1b illustrates the semiconductor substrate of FIG. 1a after viasand/or trenches are formed in the dielectric layer.

FIG. 1c illustrates the semiconductor substrate of FIG. 1b after a thinbarrier layer has been deposited thereon.

FIG. 1d illustrates the semiconductor substrate of FIG. 1c after a layerof Copper material has been deposited thereon.

FIG. 1e illustrates the semiconductor substrate of FIG. 1d afterchemical mechanical polishing of the excess copper layer and barrierlayer.

FIG. 2 illustrates one embodiment of a scrubber system.

FIG. 3 illustrates a flowchart of one embodiment of the process of thepresent invention.

DETAILED DESCRIPTION

A cleaning solution and a method for cleaning semiconductor substratesafter polishing of copper film are disclosed. In the followingdescription, numerous specific details are set forth such as specificmaterials, processes, parameters, dimensions, etc. in order to provide athorough understanding of the present invention. It will be obvious,however, to one skilled in the art that these specific details need notbe employed to practice the present invention. In other instances, wellknown materials or methods have not been described in detail in order toavoid unnecessarily obscuring the present invention.

The following description describes a cleaning solution, methods, andapparatus for cleaning a semiconductor substrate after the formation ofcopper interconnect(s) and chemical mechanical polishing(CMP)/planarization of that copper interconnect(s). It should be notedthat the processes for formation of copper interconnects insemiconductor device fabrication are well known in the art and aretherefore not described in detail herein.

It should also be noted that, although the term semiconductor substrateis used throughout this disclosure and claims the term semiconductorsubstrate is used to refer to a silicon semiconductor substrate or apart thereof, such as gallium arsenide, upon which device layers havebeen or are going to be formed. It should also be noted that the termsubstrate includes but is not limited to: fully processed,semi-processed, or unprocessed substrates with semiconductor materialsthereon.

Additionally it should be noted that, although the present invention isdescribed in conjunction with the scrubbing of a semiconductor substrateor wafer, it will be appreciated that any similarly shaped, i.e.generally flat substrate, may be processed by the methods andapparatuses of the present invention. Further, it will be appreciatedthat reference to a semiconductor substrate or wafer may include a bareor pure semiconductor substrate, with or without doping, a semiconductorsubstrate with epitaxial layers, a semiconductor substrate incorporatingone or more device layers at any stage of processing, other types ofsubstrates incorporating one or more semiconductor layers such assubstrates having semiconductor on insulator (SIO) devices, orsubstrates for processing other apparatuses and devices such as flatpanel displays, multichip modules, etc.

As described in the background of the invention after the copperinterconnects on a semiconductor substrate have been planarized usingCMP techniques, it is necessary to clean the semiconductor substrate andremove any contaminants from the surface and subsurface of thesemiconductor substrate. One such technique for removing contaminantsfrom the semiconductor substrate is scrubbing the semiconductorsubstrate (substrate).

As an example, and not by limitation, the present invention is describedin conjunction with a scrubbing process, more specifically, a scrubbingprocess in which both sides of the wafer are scrubbed simultaneously.The scrubber may include a number of stations. Each of these stationsrepresents one or more steps in the substrate cleaning process.Contaminated substrates are loaded at one end of the system and cleanedand dried substrates are unloaded from the other end of the system.Example of a systems of this type are the DSS-200™ Scrubber and theSynergy™ Scrubber available from Lam Research Corporation of Fremont,Calif.

FIG. 2 represents a cross sectional view of a Synergy™ configuration(cleaning system). Usually, the contaminated substrates are delivered tothe cleaning system after chemical mechanical planarization (CMP), froma wet bench, or from other processes resulting in contamination. At thestart of the cleaning process contaminated substrates are loaded into awafer cassette 280 (cassette) and the cassette 280 is then placed intothe wet send indexer station 210. After cassette 280 is placed into wetsend indexer station 210, the substrates are automatically removed fromthe cassette 280 and placed, one at a time, into the outside brushstation 220.

In the outside brush station 220, a substrate is processed through afirst scrub. During the first scrub, the cleaning solution may beapplied to the substrate in several different ways. For example, in oneembodiment the cleaning solution is sprayed onto the substrate. Inanother embodiment the cleaning solution is applied to the substratethrough brushes 221. Yet another embodiment applies the cleaningsolution by dripping the cleaning solution onto the substrate.

The scrubbed substrate is then automatically removed from the outsidebrush station 220 and placed into the inside brush station 230. In theinside brush station 230, the substrate is processed through a secondscrub. In the inside brush station 230 the cleaning solution may beapplied to the substrate in a similar manner as in outside brush station220.

After the second scrub the substrate is then automatically removed fromthe inside brush station 230 and placed into the rinse, spin and drystation 240. Rinse, spin, and dry station 240 rinses, spins, and driesthe substrate. At this point the wafer has been cleaned.

Once the rinse, spin, and dry steps have been completed the substrate isthen transported from the rinse, spin, and dry station 240 to the outputstation 250 where the substrate will be placed into cassette 281. Thetransfer is usually carried out by a robotic arm which lifts thesubstrate out of the rinse, spin, and dry station 240 by its edges andplaces it into the cassette 281. The cassette is then transferred tostorage or to another cleaning or processing system.

It will be clear to one of ordinary skill in the art that some of thesteps in the cleaning system described above may occur in another orderand/or with various solutions depending upon the substrate or substratelayer being cleaned. For example, different cleaning solutions, such aswater, citric acid, ammonium hydroxide, ammonium citrate, andhydrofluoric acid solution (or mixtures of solutions) may be used in oneof the brush stations. Also, other systems may include one brushstation, or more than two brush stations. Moreover, other systems mayomit one or more of the above stations/steps and may include additionalprocessing stations, such as a CMP station.

It should be noted that while the following description illustrates theuse of the present invention in a cleaning system in which both sides ofthe substrate are scrubbed simultaneously, the present invention may beused in other cleaning systems and processes. For example, a cleaningsystem in which only a single side of the substrate is scrubbed or acleaning system in which the substrate is cleaned with chemical spray.

FIG. 3 illustrates one embodiment of the process of the presentinvention. At step 310, the copper layer is planarized using chemicalmechanical polishing. It should be noted that other techniques forplanarization of the copper layer may be used and that it may still bedesirable to clean the semiconductor substrate using the presentinvention after such planarization in order to remove potentialcontaminants from the substrate surface and/or subsurface.

At step 320, the polished semiconductor substrate is then placed in ascrubber. The substrate is then scrubbed, at step 330, to remove thecontaminants caused by the polishing process. During scrubbing, acleaning solution is applied to the substrate in order to aid and/oreffectuate the removal of the contaminants (step 340). This cleaningsolution may be used in either outside brush station 220 or inside brushstation 230, or both brush stations if necessary, of the scrubber inFIG. 2.

In one embodiment, the present invention uses a cleaning solution thatis made up of deionized water, an organic compound, and a fluoridecompound, all of which are combined in an acidic pH environment forcleaning the surface of a semiconductor substrate after polishing acopper layer. The use of an acidic pH environment helps dissolve copperoxide and alleviates some of the problems of brush loading discussed inthe background of the invention. It should be noted that it is desirableto keep the acidic pH environment within a pH level range ofapproximately 1-6. In one embodiment of the present invention, theacidic pH environment has a pH level in the range of approximately 2-4.

The use of an organic compound helps to change the electrostatic forcesbetween the particles and surfaces of the brush and substrate in orderto make them repulsive. Thus, the particles repel the brushes and thesubstrate and the substrate and brushes repel the particles providingfavorable conditions for particle removal. The organic compound used maybe an organic acid, the ammonium salt of an organic acid, or an anionicsurfactant. Some examples of potential organic acids may be: citricacid, malic acid, malonic acid, succinic acid, or any combination ofsuch organic acids.

It should be noted that it is desirable to dissolve the organic compoundin deionized water (DIW) in a concentration range of approximately 100ppm to 2% by weight. In one embodiment of the present invention theorganic compound is dissolved in DIW such that the concentration rangeis approximately 200 ppm to 0.2% by weight.

The use of a fluoride compound helps to remove the contaminants from thesurface and subsurface of the dielectric layer. The present inventionincorporates the use of fluoride compounds such as hydrogen fluorideacid (HF) or ammonium fluoride (NH₄F) in the cleaning solution. HF orbuffered HF (ammonium fluoride mixed with HF) etches silicon dioxide.Thus, during the scrubbing process the silicon dioxide layer is alsoetched. For example during the brush scrubbing process, typicallybetween 10-100 Å of the oxide layer is removed, leaving clean,uncontaminated dielectric surface on the substrate.

It should be noted that it is desirable to dissolve the fluoridecompound in deionized water (DIW) in a concentration range ofapproximately 0.1% to 5% by weight. In one embodiment of the presentinvention the fluoride compound is dissolved in DIW such that theconcentration range is approximately 0.2% to 1% by weight.

The cleaning solution of the present invention is a mixture of chemicalsin DIW containing a fluoride compound such as HF or NH₄F; an organicacid, an ammonium salt of an organic acid, or an anionic surfactant; inan acidic pH environment. It should be noted, however, that hydrochloricacid (HCl) may also be added to the solution to adjust pH and helpdissolve copper oxide. It should also be noted that the chemicals of thepresent invention are pre-mixed in the same cleaning solution tosimultaneously solve several problems related to post copper CMPcleaning using a brush scrubber. Cross contamination from substrate tosubstrate and within the same substrate are therefore reducedsubstantially, or even prevented in this simple approach.

One example of the many different ways in formulating the cleaningsolution is: 0.5% HF, 0.1% Citric Acid, and 0.4% NH₄OH by weight mixedin DIW. The pH level of the solution in this example is approximately 3.It should be noted however, that the present invention covers variousformulations of the cleaning solution, and that each component in thesolution may be replaced by different chemical that has similarproperties.

Another exemplary way to formulate the cleaning solution is by mixingcitric acid in a concentration range of approximately 0.09%-0.11% byweight, ammonium fluoride in a concentration range of approximately 0.4%-0.6% by weight, and hydrogen fluoride in a concentration range ofapproximately 0.09%-0.11% by weight in DIW. The resultant cleaningsolution has a pH of about 4. In one embodiment, the cleaning solutionis formed by mixing about 0.1% by weight of citric acid, about 0.5% byweight of ammonium fluoride and about 0.1% by weight of hydrogenfluoride in DIW.

It will be apparent to those skilled in the art that numerous chemicalreactions will take place between the starting materials when they aremixed in DIW. For example, in the case of a cleaning solution formed bymixing about 0.1% by weight of citric acid, about 0.5% by weight ofammonium fluoride by weight, and about 0.1% by weight of hydrogenfluoride in DIW, the thus-formed solution will contain at least thefollowing species: fluoride ions, ammonium ions, citrate ions, citricacid, and hydrogen fluoride. The relative percentages of these speciesfor this exemplary cleaning solution have been calculated to be asfollows (assuming room temperature (25° C.): about 0.5% by weight ofammonium (NH₄ ⁺) ions, about 0.45% by weight of fluoride (F) ions, about0.15% by weight of hydrogen fluoride (HF), about 0.09% by weight ofcitrate ions, and about 0.01% by weight of citric acid. Those skilled inthe art will appreciate that the relative amounts of these species mayvary at temperatures other than room temperature.

For ease of transportation from the place of manufacture to the place ofuse, the above formulations of the cleaning solution may be concentratedand diluted later to the proper concentration by the end user at thesite of use, e.g., proximate to a scrubbing apparatus. Moreover,diluting the concentrated cleaning solution with DIW reduces thefrequency with which the container from which the cleaning solution isdispensed must be replaced in a semiconductor fabrication facility. Forexample, one exemplary concentrated cleaning solution may be produced bymixing citric acid in a concentration range of approximately 1.8%-2.2%by weight, ammonium fluoride in a concentration range of approximately8%-12% by weight, and hydrogen fluoride in a concentration range ofapproximately 1.8%-2.2% by weight in DIW. The resultant concentratedcleaning solution has a pH of about 3.8. In one embodiment, theconcentrated cleaning solution is formed by mixing about 2% by weight ofcitric acid, about 10% by weight of ammonium fluoride, and about 2% byweight of hydrogen fluoride in DIW. In one embodiment, the concentratedcleaning solution is diluted by mixing one unit volume of theconcentrated cleaning solution with 20 unit volumes of DIW.

If desired, other suitable organic acids may be substituted for citricacid in the cleaning solution formulations described above. By way ofexample, suitable organic acids include malic acid, malonic acid,succinic acid, oxalic acid, or any combination thereof.

Thus, methods and apparatus for cleaning semiconductor substrates afterpolishing of copper film have been described. Although specificembodiments, including specific equipment, parameters, methods, andmaterials have been described, various modifications to the disclosedembodiments will be apparent to one of ordinary skill in the art uponreading this disclosure. Therefore, it is to be understood that suchembodiments are merely illustrative of and not restrictive on the broadinvention and that this invention is not limited to the specificembodiments shown and described.

What is claimed is:
 1. A cleaning solution formed by mixing an amount ofcitric acid, an amount of ammonium fluoride, and an amount of hydrogenfluoride in deionized water, the amount of citric acid being in a rangefrom about 0.09% by weight to about 0.11% by weight, the amount ofammonium fluoride being in a range from about 0.4% by weight to about0.6% by weight, the amount of hydrogen fluoride being in a range fromabout 0.09% by weight to about 0.11% by weight, and the cleaningsolution having a pH of about
 4. 2. The cleaning solution of claim 1,wherein the amount of citric acid is about 0.1% by weight, the amount ofammonium fluoride is about 0.5% by weight, and the amount of hydrogenfluoride is about 0.1% by weight.
 3. A concentrated cleaning solutionfor cleaning semiconductor substrates, the concentrated cleaningsolution being formed by mixing an amount of citric acid, an amount ofammonium fluoride, and an amount of hydrogen fluoride in deionizedwater, the amount of citric acid being in a range from about 1.8% byweight to about 2.2% by weight, the amount of ammonium fluoride being ina range from about 8% by weight to about 12% by weight, the amount ofhydrogen fluoride being in a range from about 1.8% by weight to about2.2% by weight, and the concentrated cleaning solution having a pH ofabout 3.8.
 4. The cleaning solution of claim 3, wherein the amount ofcitric acid is about 2% by weight, the amount of ammonium fluoride isabout 10% by weight, and the amount of hydrogen fluoride is about 2% byweight.
 5. A method for cleaning a semiconductor substrate having apolished copper layer comprising: providing a concentrated cleaningsolution formed by mixing an amount of citric acid, an amount ofammonium fluoride, and an amount of hydrogen fluoride in deionizedwater, the amount of citric acid being in a range from about 1.8% byweight to about 2.2% by weight, the amount of ammonium fluoride being ina range from about 8% by weight to about 12% by weight, the amount ofhydrogen fluoride being in a range from about 1.8% by weight to about2.2% by weight, and the concentrated cleaning solution having a pH ofabout 3.8; placing the semiconductor substrate having a polished copperlayer in a scrubbing apparatus; mixing one unit volume of theconcentrated cleaning solution with 20 unit volumes of deionized waterproximate to the scrubbing apparatus to obtain a cleaning solutionhaving a pH of about 4; and scrubbing the semiconductor substrate in thescrubbing apparatus in the presence of the cleaning solution.